[aheilbut]

I think it will be a lot like email - what is undesirable about appending a domain? That's how the internet works.

[alextgordon]

You mean, with all the spam?

[wmf]

Fortunately tweeting has the concept/expectation of following (whitelisting) and email doesn't.

[alextgordon]

Only if you've protected your profile/using private messages. I regularly get @replied by people who I don't follow, and I don't particularly want to lose those messages, so it's probably not a realistic way of stopping spam (although it would certainly be effective).